1. Field of the Invention
This invention relates to a mask unit for paste printing for forming patterns by use of an ink, an adhesive, solder paste, or a paste-like resin on a printing material, and more particularly to a plastic mask unit which has a long life and is capable of performing paste printing of a superior quality.
This invention also relates to a method of fabricating the above-mentioned plastic mask unit.
2. Discussion of the Background
In the field of paste printing, several different types of conventional masks have been put in practice for forming printed patterns on a printing material by use of various pastes, such as, solder paste, an ink, an adhesive agent, and a paste resin (hereinafter referred to as the paste). An example of conventional printing masks for the paste printing is a mask known as a punch press mask which can be prepared by forming round through-holes in a metal sheet with a punch. Another example is a YAG laser mask which can be prepared by forming through-holes in a metal sheet with a YAG laser. Another example is an etching mask which can be prepared by forming through-holes in a metal sheet using a metal etching process. Still another example is an additive mask which can be prepared by forming through-holes in a metal plate formed by being plated around a pattern of through-holes.
In addition to the above-mentioned metal masks, a mask made of a plastic sheet has been recently introduced which is disclosed in the official gazette for Japanese Laid Open Patent Application TOKUKAI HEI 7-81027. This plastic mask can be prepared by forming slit-shaped through-holes in a plastic sheet by excimer laser abrasion. As a result, the plastic mask becomes free from various known shortcomings, such as, for example, burr in the edges of the through-holes, deposition of dross in the through-holes, and so forth, that occur in the conventional metal masks. Also, such a plastic mask meets increasing demands for highly precise paste printing of a micro pattern with high quality, which arise from the recent trend whereby electronic appliances and parts are made smaller and smaller in size.
In these conventional masks, a metal sheet or a plastic sheet is secured to a mask frame with a mesh screen inserted in between. A fabrication procedure for these conventional masks is now explained with reference to FIGS. 10(a)-10(h).
In FIG. 10(a), reference numeral 301 denotes a mask frame to which a rectangular cross section aluminum pipe is commonly adopted. Such an aluminum mask frame is coated with a coating agent suitable for gluing aluminum so that a first thin film 302 suitable for gluing aluminum is formed in a square frame on the edge of the mask frame 301 as shown in FIG. 10(a). A rubber adhesive diluted with a diluent is commonly applied to this coating material. A rough finish may be provided to the edge of the mask frame 301 before the first thin film 302 is formed thereon.
In a similar manner, a mesh screen 303 is coated with a coating material suitable for gluing polyester, for example, so that a second thin film suitable for gluing polyester is formed in a square frame on the mesh screen 303. For the mesh screen 303, either a polyester mesh screen of about 180 to 225 mesh or a nylon mesh screen of about 180 to 225 mesh can be applied.
Next, a step of gluing the polyester mesh screen 303 to the mask frame 301 is illustrated in FIG. 10(b). An example of an adhesive is an epoxy type adhesive which is suitable for gluing both the first thin film formed on the mask frame 301 and the second thin film formed on the polyester mesh screen 303. Such an epoxy adhesive is coated on either one or both of the first thin film formed on the mask frame 301 and the second thin film formed on the polyester mesh screen 303. After that, the polyester mesh screen 303 is affixed to the mask frame 301 with the application of a predetermined tension thereto in the directions as indicated by arrows in FIG. 10(b).
As a result, the mesh screen 303 is secured to the mask frame 301 as shown in FIG. 10(c). Since, in addition to extension caused by the predetermined tension, the mesh screen 303 is prepared in a predetermined size larger than the mask frame 301, there are overhanging edges 303a of the mesh screen relative to the periphery of the mask frame 301 as shown in FIG. 10(c). Accordingly, these overhanging edges 303a of the mesh screen 303 are cut off so that the mesh screen 303 becomes equal in size to the mask frame 301 in the horizontal plain in the drawing.
Next, a step of drawing register marks for accurately registering the mesh screen relative to the mask plate is shown in FIG. 10(d). A register mark 303b is drawn at the centers of all four sides of the mesh screen 303 secured to the mask frame 301. The register marks 303b are drawn softly and visibly using a sharp-pointed pencil on the mesh screen 303 so that the mask plate 304 can be accurately registered relative to the mesh screen 303 in the following step.
The mask plate 304 is then coated with a coating agent as shown in FIG. 10(e). This mask plate 304 includes a number of slit-shaped through-holes for serving as a print pattern for paste printing. The edge of the mask plate 104 is coated with a coating agent suitable for gluing the mask plate 304. As a result, a thin film 304b suitable for gluing the mask plate 304 is formed. Then, the mask plate 304 is secured to the mesh screen 303. In this event, a rubber type adhesive diluted with a diluent is usually applied to the coating agent. In addition, the edge frame of the mesh screen 303 may be coated with another coating agent suitable for gluing polyester, for example, so that a polyester thin film suitable for gluing polyester can be formed on the edge frame of the mesh screen 303. Moreover, a rough finish or a number of fine holes may be provided at the edge of the mask plate 304 before the thin film 304b is formed. thereon.
Next, the step of securing the mask plate 304 to the mesh screen 303 on the mask frame 301 is shown in FIG. 10(f). In this step, the position of the mask plate 304 on the mesh screen 303 is manually adjusted so that center holes 304c on each side of the mask plate 304 and the register marks 303b of the mesh screen 303 can be finely registered. Then, the mask plate 304 is temporarily taped to the mask frame 301 and the mesh screen 303 to form an integrated unit. After that, this one unit of the mask frame 301, the mesh screen 303, and the mask plate 304 is turned upside down and masking tape is placed over a relatively large area of the mesh screen 303, leaving an area in the form of a square frame to be coated with adhesive, while the taped area is not coated with the adhesive. This masking tape must be in place before an epoxy type adhesive, for example, is coated onto the square frame of the mesh screen 303. The masking tape is removed after the adhesive is cured on the mesh screen 303.
The step of making an opening 303c is explained in FIG. 10(g). The opening 303c is formed by cutting out the mesh screen 303. The area cut must not be secured by the mask plate 304 so that a portion 305 in which the adhesive is cured on the mesh screen 303 can be left. After that, each glued portion is provided with guard tapes to prevent peeling-off so that the mask frame 301 and the mesh screen 303, the mask plate 304 and the mesh screen 303 are mutually secured.
In this way, the conventional mask unit for paste printing is fabricated. Consequently, the thus fabricated printing mask unit has a three-layer structure wherein the mask frame 301 and the mask plate 304 are secured with the mesh screen 303 inserted in between, as shown in FIGS. 10(g) and 10(h).
However, there has been a problem in which registration of through-holes relative to the printing material becomes inaccurate in the case where a plastic sheet is applied to the mask plate during the above-mentioned fabrication procedure of the mask unit for paste printing. This is because of the tension, which is applied to the mesh screen secured to the mask frame, for holding the mask plate at a relatively high degree of flatness when the mask plate is secured to the mask frame. Accordingly, when a plastic sheet is used as the mask plate, this plastic mask plate may be overstretched by the above-mentioned tension and thereby registration of through-holes relative to the printing material may deteriorate.
There has been another problem in the above-mentioned conventional mask unit for paste printing in which the life of the mask unit becomes relatively short. This is because two secured portions of the conventional mask unit, one between the mask frame and the mesh screen and the other between the mask plate and the mesh screen, are breakable and apt to be peeled off when the mask unit is subjected to washing for a number of times to protect the through-holes from being filled with paste material. In this case, if the secured portions are peeled off, the mask plate loses tension so that the position of the through-holes relative to the printing material is displaced, resulting in deterioration of print quality.
Further, there has been another problem in the conventional mask unit for paste printing and its fabrication procedure. This is an increase in fabrication steps, such as, for example, the step of placing a masking tape on the mesh screen, the step of cutting off the mesh screen, and so forth.
Further, there has been another problem in the conventional mask unit for paste printing and its fabrication procedure. This is the problem of displacement of the mask plate relative to the mask frame. This is because the steps for securing the elements are separated into one step for the mask frame and the mesh screen and another step for the mask plate and the mesh screen. The resultant displacement causes inaccurate registration of the through-holes of the mask plate relative to the printing material, leading to a deterioration of print quality. Moreover, owing to this problem, there has been another problem in which fine adjustment of the position of the mask plate is required during installation of the mask unit on a paste printing machine.